In the air oxygen/nitrogen equipment, whether the forced valve or the automatic valve is not closed tightly, it will cause medium and low pressure blow-by, which will increase the air flow and reduce the air pressure. To distinguish whether the problem is caused by the failure of the automatic valve or the failure of the forced valve, it can be judged from the following three aspects.

1) The automatic valve is not closed tightly, the gas from the upper tower is difficult to discharge, and the pressure of the low-pressure system increases. When the forced valve is not tight, the pressure on the tower is usually reduced.

2) When the automatic valve is not closed tightly, the temperature of the channel (or the regenerator) that takes the air flow increases due to the increase of the air volume, and the temperature of the middle of the dirty nitrogen channel decreases due to the increase of the return flow.

3) The failure of the automatic valve will reduce the purity of the reflux gas in the switching heat exchanger, while the failure of the forced valve only affects the purity of the reflux gas out of the device.

Based on this, the gas purity of the three places before the automatic valve, the front and the rear of the forced valve are separately tested, and then the source of the fault is judged according to the change of the purity.